Concrete Technology (CE5G) - Special types of concrete.ppt

composed essentially of portland cement and water, coats the surface of the fine (small) and coarse (larger) aggregates. Through a series of chemical reactions called hydration, the paste hardens and gains strength to form the rock-like mass known as concrete.

Within this process lies the key to a remarkable trait of concrete: it's plastic and malleable when newly mixed, strong and durable when hardened. These qualities explain why one material, concrete, can build skyscrapers, bridges, sidewalks and superhighways, houses and

1. PRESTRESSED
CONCRETE STRUCTURES
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2. Reinforced concrete:
•Concrete is strong in compression weak in tension.
•Steel in strong in tension
•Reinforced concrete uses concrete to resist compression
and to hold bars in position and uses steel to resist tension.
•Tensile strength of concrete is neglected (i.e. zero )
•R.C beams allows crack under service load.

3. Pre-stressed Concrete
•What is Pre-stressed Concrete?:
–Internal stresses are induced to counteract external
stresses.
–In 1904, Freyssinet attempted to
introduce permanent acting forces
in conc. to resist elastic forces
under loads and was named
“Pre stressing”.

5. Principle of pre-stressing:
•Pre-stressing is a method in which compression force is
applied to the reinforced concrete section.
•The effect of pre stressing is to reduce the tensile stress in the
section to the point till the tensile stress is below the cracking
stress. Thus the concrete does not crack.
•It is then possible to treat concrete as a elastic material.
•The concrete can be visualized to have two compressive force
i . Internal pre-stressing force.
ii . External forces (d.l , l.l etc )
•These two forces must counteract each other.

6. Components of Pre-stressing:
Concrete –M30 to M60 grade
Reinforcement-steel-High tensile strength steel
wires called as cables or tendons.
Hard drawn wires of dia 1.5 mm to 8 mm used as
single wire or cable. OR
High steel alloy bars 9.5 mm to 32 mm

7. Pre-stressed Concrete: Methods
•There are two basic methods of applying pre-stress to a
concrete member
–Pre-tensioning –most often used in factory situations
–Post-tensioning –site use

8. Types of pre-stressing:
I . Pre-tensioning
In Pre-tension, the tendons are tensioned against some
abutments before the concrete is place. After the concrete
hardened, the tension force is released. The tendon tries to
shrink back to the initial length but the concrete resists it through
the bond between them, thus, compression force is induced in
concrete. Pretension is usually done with precast members

10. II . Post tensioning
•In Post tension, the tendons are tensioned after the
concrete has hardened. Commonly, metal or plastic
ducts are placed inside the concrete before casting.
After the concrete hardened and had enough strength,
the tendon was placed inside the duct, stressed, and
anchored against concrete. Grout may be injected into
the duct later. This can be done either as precast or
cast-in-place.

11. Post tensioning

13. Advantages:
•Take full advantages of high strength concrete
and high strength steel
•Need less materials
•Smaller and lighter structure
•No cracks
•Use the entire section to resist the load
•Better corrosion resistance
•Good for water tanks and nuclear plant
•Very effective for deflection control
•Better shear resistance

14. Disadvantages compared to RC:
•Need higher quality materials
•More complex technically
•More expensive
•Harder to re-cycle

15. Application:
•Bridges
•Slabs in buildings
•Water Tank
•Concrete Pile
•Thin Shell Structures
•Offshore Platform
•Nuclear Power Plant
•Repair and Rehabilitations

16. POST TENSIONING

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